1. Field of the Invention
The present invention relates to a porous sintered body, a method of manufacturing a porous sintered body, and a method of manufacturing an exhaust gas purifying apparatus.
2. Discussion of the Background
In recent years and continuing, there is a problem in which particulates contained in exhaust gas of internal combustion engines of various vehicles (e.g. buses, trucks) and machinery (e.g. construction machinery) adversely affect the environment and the human body.
Accordingly, there are proposed various honeycomb structural bodies formed of porous silicon carbide material for use as a filter that purifies exhaust gas by collecting particulates contained in the exhaust gas of internal combustion engines. One example is a filter having a honeycomb structural body including numerous porous pillar-shaped cells partitioned by cell walls and extending in a longitudinal direction. Since each cell of the honeycomb structural body has one of its ends sealed with a sealing material, the exhaust gas guided into the honeycomb structural body can only be discharged out of the honeycomb structural body by passing through the cell walls of the honeycomb structural body. The particulates or the like contained in the exhaust gas are collected when the exhaust gas passes through the cell walls. Furthermore, since harmful gas components contained in the exhaust gas (e.g. CO, HC, and NOx) can be purified by a catalyst reaction, there is another proposed example in which the foregoing honeycomb structural body has catalyst carried by its cell walls.
It is to be noted that (the cell walls of) the foregoing honeycomb structural body has a high porosity (for example, 45-60%). This is due to the fact that a low porosity increases the amount of damage caused by pressure when the exhaust gas flows through the honeycomb structural body. This applies, particularly, to a honeycomb structural body carrying a large amount of catalyst since a part of the pores of the cell walls may become clogged when the catalyst is carried in the cell walls. Thus, a higher porosity is desired.
In order to attain a honeycomb structural body having a high porosity, PCT International Publication No. 2002-096827 discloses a method of fabricating a honeycomb structural body by adding a pore forming material to a ceramic particle raw material beforehand and eliminating the pore forming material during a sintering process. The contents of this publication are incorporated by reference in their entirety.
However, in the case of adding a pore forming material to a ceramic particle material and conducting a sintering process, the obtained honeycomb structural body exhibits a pore diameter distribution C having a broad peak as shown in FIG. 1. This is because the pore diameter distribution C illustrates the total sum of a pore diameter distribution A obtained under the same conditions where a ceramic particle raw material without a pore forming material is sintered and a pore diameter distribution B obtained where pores are formed by a pore forming material. With a honeycomb structural body exhibiting a pore diameter distribution as C in FIG. 1, reduction of collection efficiency cannot be prevented even when the average pore diameter is controlled within a desired range.
In order to prevent such a problem, PCT International Publication 2002-026351 discloses a method of narrowing the obtained pore distribution by matching the particle diameter of the pore forming material with the pore diameter of the final product obtained after sintering only the ceramic particle raw material such that the pore distribution B obtained where pores are formed by the pore forming material after the sintering process is matched with the pore distribution A where a ceramic particle raw material is sintered. The contents of this publication are incorporated by reference in their entirety.